1. Field of the Invention
This invention relates to a process for producing silanes represented by the general formula Si.sub.n H.sub.2n+2 wherein n is 1 or 2 by reacting an alloy containing silicon with an acid.
2. Description of the Prior Art
With a tremendous advance in the electronics industry in recent years, there has been a rapidly increasing demand for silicon for semiconductors such as polycrystalline silicon or amorphous silicon. Silanes have recently gained increasing importance as a material for production of such silicon for semiconductors. In particular, silane (SiH.sub.4) and disilane (Si.sub.2 H.sub.6) are expected to have a greatly increased demand in future as a material for solar cell semiconductors, electrophotographic materials, thin-film transistors, etc.
Particularly, Si.sub.2 H.sub.6 has a much higher deposition rate than SiH.sub.4 in the formation of an amorphous silicon film by chemical vapor deposition (CVD) [for example, Appl. Phys. Lett., 37, 725 (1980)], and has recently been rapidly studied for new uses as a new starting gas for formation of semiconductor films.
Some methods illustrated below have previously been known for the production of silanes. ##STR1##
Of these, the methods (1) and (2) involving reaction of a silicon alloy, particularly magnesium silicide, with an acid have long been known as most easily practicable methods. The methods (1) and (2) have the advantage that they do not require expensive reducing agents as does the reaction of the method (3), and they do not require reaction under elevated pressures as does the reaction of the method (4) but can be carried out substantially at room temperature and atmospheric pressure. For example, disilane (Si.sub.2 H.sub.6) can be obtained by reducing expensive hexachlorodisilane (Si.sub.2 Cl.sub.6) with a metal hydride. But it can be very easily obtained by the method (1) or (2), particularly the method (1). In the method (1) in which the reaction is carried out in a water solvent, formation of a silicon compound having a siloxane linkage as a by-product cannot be avoided. Consequently, the conversion of silicon in the silicon alloy to silanes (to be referred to as the yield based on silicon atom) is low, and the ratio of SiH.sub.4 to Si.sub.2 H.sub.6 formed is invariable the total yield of SiH.sub.4 and Si.sub.2 H.sub.6 is about 30%; the SiH.sub.4 /Si.sub.2 H.sub.6 mole ratio is about 2 (based on Si atoms); see, for example, Journal of the Chemical Society, 1131 (1946)]. The method (1) also has the disadvantage that since a viscous black solid accumulates in the reactor as the reaction proceeds, its adhesion to the reactor wall reduces thermal conduction, and makes the stirring of the reaction mixture poor. Furthermore, according to the method (1), self-combustible gas is handled in a strongly corrosive acidic aqueous solution. Hence, a technically high level of measures for safety is required.
The present inventors made extensive efforts in order to solve this problem, and previously found that the yields of SiH.sub.4 and Si.sub.2 H.sub.6 are increased greatly by a method involving the copresence of an organic solvent such as an ether or a hydrocarbon in the reaction system, or a method by which by-product higher silanes soluble in the aforesaid organic solvent are converted to lower SiH.sub.4 and Si.sub.2 H.sub.6 (the total yields of SiH.sub.4 and Si.sub.2 H.sub.6 is 60 to 70%, for example, Japanese Laid-Open Patent Publications Nos. 141614/1985, 141615/1985, 255613/1985, and 251114/1985). However, even by these methods, it is difficult to vary the ratio of SiH.sub.4 to Si.sub.2 H.sub.6 as desired, and the SiH.sub.6 /Si.sub.2 H.sub.6 mole ratio is within a narrow range of about 1 to 2 (Si atom base).
The method (2) is a highly safe process without a risk of corrosion as in the method (1), and the yield of SiH.sub.4 is 70 to 80%. The yield of Si.sub.2 H.sub.6, however, is very low and is 5% at most (for example, Journal of American Chemical Society, vol., 57, 1349 (1935); Japanese Patent Publication No. 14708/1967; and Japanese Patent Publication No. 22918/1973).
Until very recently, however, only SiH.sub.4 had been substantially used in the silicon semiconductor industry, and the main purpose had been to produce SiH.sub.4. Accordingly, no work on increasing of the yield of Si.sub.2 H.sub.6 has previously been done, and no method has previously been known for varying the ratio of SiH.sub.4 to Si.sub.2 H.sub.6 as desired.
It has recently been made clear that in the formation of amorphous silicon film by, for example, CVD, disilane has a much higher deposition rate than monosilane, and disilane has been rapidly studied for utility as a new material gas for formation of semiconductor films which supersedes monosilane.
Accordingly, it is very desirable to use monosilane or disilane selectively as a starting gas in the production of silicon for semiconductors so that they exhibit their inherent characteristics as much as possible.